1. Field of the Invention
The present invention relates to an improved vapor phase process for the catalytic oxidation of propylene to acrylic acid in a single step or stage using as oxidant a mixture of solids in an oxidized state, and where the resulting reduced solids is separately regenerated using molecular oxygen. More specifically but not by way of limitation, the invention relates to a process for performing this reaction in a recirculating solids reactor system.
2. Description of the Related Art
An important route to acrylic acid is the vapor phase oxidation of propylene over a multicomponent catalyst containing molybdenum and/or other metals, usually as their oxides. Typically, this is carried out in two steps. The first reaction step involves oxidation of propylene with air (oxygen) to form acrolein, often with a minor amount of acrylic acid, along with carbon oxides, water and smaller amounts of other oxidized byproducts. The second reaction step then converts acrolein to acrylic acid by a similar oxidation step, but typically using different reaction conditions and catalyst for optimum results.
In some proposed processes, the amount of acrylic acid co-produced with acrolein is large enough to merit isolating it as product, and recycling the acrolein back to the oxidation step. These processes typically require the separation and recycle of large amounts of acrolein.
Typically these reactions are carried out in multitubular fixed-bed reactors. The large exothermic heat of reaction and the thermal sensitivity of the propylene oxidation requires low feed concentrations, expensive heat transfer equipment, handling of a large volume of gas, and good reactor temperature control. Low propylene concentration is also required to avoid flammability conditions.
The magnitude of some of these problems is reduced when fluidized-bed reactors are used. The temperature can be readily controlled within a few degrees because of the intensive catalyst mixing and the good heat transfer characteristics. Higher propylene concentrations can be used because the danger of flammability is reduced by introducing the propylene directly into the reactor rather than premixing it with air (oxygen). However, very high propylene concentrations and low oxygen-to-propylene ratios in the reactor may result in the over reduction of the solids and reduced selectivity to the desired products. Also, significant back-mixing of gases in the fluidized-bed reactor results in poorer selectivity and makes it difficult to obtain high propylene conversion.
Modified forms of fluidized-bed reactor are known as recirculating solids reactor, transport bed reactor, transport line reactor, riser reactor, fast fluidization reactor, multi-chamber fluidized bed reactor, and by other names, depending on design and/or personal preference. In this application we will use the term "transport bed reactor" to mean any reactor in which solid particles are injected at one end of the reactor and carried along with gas reactants at high velocities and discharged at the other end of the reactor to a gas-solids separation vessel. A riser reactor, in which the reactor is a vertical pipe wherein the active solids and gases are fed in at the bottom, transported in essentially plug flow and removed at the top, is one example of a transport bed reactor. Another example is a pipeline reactor, in which the flow of active solids and gases is other than vertically upwards. A transport bed reactor, as defined herein, includes a riser reactor or pipeline reactor which also incorporates a zone for fluidization, i.e., a zone where the gas velocities are sufficiently high to carry out a substantial portion of the active solids fed, but with more back-mixing of active solids than would occur in plug flow. We will use the term "recirculating solids reactor system" to mean a general reaction system with two reaction zones, in which two separate reactions take place, and which uses a particulate solid which circulates between the two reaction zones and takes part in both reactions. Optionally, either or both reaction zones may involve either a transport bed reactor or a fluidized bed. Such reaction systems have found use in catalytic cracking in petroleum refining and in other reactions.
U.S. Pat. No. 4,668,802 discloses a process for preparing maleic anhydride by oxidizing butane using an oxidized vanadium-phosphorous oxide catalyst as oxidant rather than oxygen wherein the resulting reduced catalyst is separately regenerated, and the use of a recirculating solids reactor system for this reaction. Certain of the examples use a transport bed or riser reactor for the butane oxidation reaction.
Japanese Kokai 3-170,445 discloses a similar process for preparing a mixture of acrolein and acrylic acid by oxidizing propane using an oxidized bismuth-molybdenum catalyst or vanadium pyrophosphate catalyst as oxidant. In Example 2, a propane conversion of 55%, an acrylic acid selectivity of 65%, and an acrolein selectivity of 7% were obtained using a catalyst consisting of vanadyl pyrophosphate and tellurium oxide.
An advertising folder prepared by E.I. DuPont in 1973 titled "Chemical Technologies Worldwide" included a single sheet titled "Transport Bed Reactor Technology for Selective Processes", which described the general advantages of a transport bed or riser reactor, listing among typical applications the reaction of propylene to make acrylic acid.
None of the above references disclose the necessary information to enable the economical use of a vapor phase process for the catalytic oxidation of propylene to acrylic acid in a single step or reaction stage using as oxidant a combination or mixture of catalyst in a fluidized and oxidized state, and where the resulting reduced catalyst is separately regenerated using molecular oxygen.
The preparation of multicomponent compositions containing molybdenum, vanadium and/or other metals and their use as catalysts in oxidation processes is well known in the art. For example, U.S. Pat. Nos. 4,677,084 and 4,769,477 disclose a process for making highly attrition resistant silica-based catalysts containing molybdenum, vanadium or other metals. Numerous other patents such as U.S. Pat. No. 3,487,109, U.S. Pat. No. 3,631,099, GB 1,490,489 or JP 05,301,051 also disclose specific catalyst compositions for use in the oxidation of propylene in a fixed-bed or fluidized-bed process.